The invention relates to a method of making large structural components, preferably such as are used for manufacturing aircraft and vessels. Such components are made of fiber reinforced synthetic materials.
In a known method for producing structural components of the above mentioned type, fiber mats and/or fiber webbings and/or rovings are employed which are impregnated in or on a mold by means of liquid synthetic materials such as epoxy resins or unsaturated polyester resins. It is customary to employ positive as well as negative molds. The curing of the synthetic material takes place at room temperature or by the application of heat, whereby the individual layers may be pressed against each other, for example, by pneumatic means. The time which is available for applying the individual layers is determined by the so-called "pot time" of the liquid synthetic materials. This time is determined by the workability of the material before it hardens. If it is not possible to apply all layers during the "pot time" of a particular batch, the application of layers is continued after a new batch of synthetic resins is mixed. This prior art method is referred to as "batch method" and is very time consuming and hence it increases labor costs. The synthetic resin mixtures must be mixed by hand and therefore mixing errors are possible. It is not possible to maintain constant the ratio between fibers and synthetic materials due to the manual layer application and the manual impregnation. The laminar shearing strength among the several layers is small. Inaccurate mixing and application lead to loss in structural strength. Such loss in strength may be compensated by larger dimensions. However, larger dimensions increase both the structural weight and the costs. Where heat curing is employed, the size of the structural component is limited by the dimensions of the autoclave. Besides, when manually handling the liquid synthetic resin impregnating materials, the volatile components of such impregnating materials are liberated which are health hazards and harmful to the environment.
An advantage of the prior art batch method is seen in that very large structural components may be made where cold curing or hardening is used.
According to another known method, fiber webbings and/or mats and/or rovings are used which are pre-impregnated in the factory. The synthetic mass forming part of these materials is already mixed. The curing takes place by the application of heat. The main advantages of this method using pre-impregnated materials are lower labor costs than in the first mentioned batch method, because the impregnating is obviated. The processing of such pre-impregnated material is thus neither a health hazard nor does it adversely affect the environment. The mixing of the synthetic material in the factory reduces mixing errors and the ratio of fibers to synthetic material is constant within narrower limits. The laminar shearing strength among the layers is high. The high constancy of the mixing ratio results in uniform material strength values which permit lower safety factors and thus lower structural weights.
However, these advantages of the prior art method using pre-impregnated materials must be weighed relative to the disadvantages, such as the heat required for curing and the limitations on the size of the structural components imposed by the dimensions of an autoclave necessary for the curing. Please see U.S. Pat. Nos. 3,666,600; granted on May 30, 1972, and 3,004,295; granted on Oct. 17, 1961.
The positive or negative molds which are employed in the two just described prior art methods, are manufactured by first making wooden mold frames and by making the sheer surfaces of steel or aluminum sheet material. For this purpose it is necessary that the respective sheet metal parts have the correct curvature or spherical shape prior to their being fitted into the mold. Thus, the costs for making the molds are very high.